The present invention relates to a full bridge circuit and, more particularly, to a full bridge circuit that allows zero voltage switching and that minimizes the effects of leakage inductance without using external snubbing circuitry.
Complexity of circuitry to obtain phase modulated square wave drives is generally costly, requires a significant amount of space and results in an undesirably high failure rate. In general, fewer components in the switching section would allow for a more compact layout, minimizing radiating loop area and parasitic inductance connecting components.
As switching frequency increases, power losses in the switching device become primarily switching losses. Therefore conventionally, only lower power converters were possible at higher switching frequencies. Zero voltage switching, however, minimizes such power losses.
As demand for higher switching frequency occurs, it becomes even more important to include parasitic devices, such as diodes and capacitors, in the design process. Utilizing the parasitics of transformers and transistors can be an advantage.
The drain-source voltage of transistors should be near zero volts to minimize or to avoid turn-on losses and electromagnetic interference. An example of zero voltage switching technique used in a half bridge converter is disclosed in "Zero-Voltage-Switching Technique In High-Frequency, Off-Line Converters" by M. Jovanovic et al, third annual IEEE Applied Power Electronics Conference, Feb. 1-5, 1988 (pp. 22-32). This paper discusses a half bridge zero-voltage-switching technique to overcome limitations of a zero-current-switch quasi-resident converter. Switching turn-on losses of power switches are eliminated by shaping the transistor's voltage waveform so that the voltage reduces to zero prior to turn-on.
A snubber circuit is commonly used with each transistor in bridge circuits to prevent the simultaneous presence of high voltage and high current in the transistor junction each time it is biased off. This condition of high voltage and high current arises because the load imposed on the transistor by the transformer's primary winding is highly inductive, and the load current therefore continues to flow, even after the transistor has been biased off. The snubber circuit serves as a bypass route for this current, avoiding the transistor junction.
Snubber circuits typically include a series diode and capacitor shunting each transistor. When the transistor is biased on, the diode ensures that the capacitor does not affect the transistor's operation. When the transistor is first biased off, however, the current that previously flowed through the transistor and that continues to flow because of the load's inductance, is diverted through the diode to charge the capacitor to a predetermined voltage. Thereafter, when the transistor is again turned on, a special circuit discharges the capacitor to its original state. This discharge circuit frequently takes the form of a resistor connected in parallel with the diode, such that discharge current is routed through the resistor and transistor.
U.S. Pat. No. 4,626,980 issued to McGuire discloses a non-dissipative snubber circuit for use in a power bridge circuit having a pair of switching power transistors for controllably coupling pulses of electrical current in opposite directions through the primary winding of a power transformer. The current flowing through the transformer's primary winding is diverted to charge one of the snubber capacitors each time the associated transistor is switched off. When the transistor is again switched on, the capacitor is reinitialized by discharging it through an inductor. In a full bridge configuration, the discharge current is used to discharge a separate snubber capacitor shunting one of two additional switching power transistors connected to the opposite side of the primary winding. The snubber circuitry in the aforementioned reference is relatively complex, but zero voltage switching is not considered therein.
Another approach to providing power supply drivers with near zero voltage switching is described in "Zero-Voltage Switching In A Constant Frequency Digitally Controlled Resonant DC-DC Power Converter" by J. G. Hayes et al, third annual IEEE Applied Power Electronics Conference, Feb. 1-5, 1988 (pp. 360-367). A resonant converter comprises resonating inductance and capacitance elements that store all of the energy that eventually is transferred to a load. In the range between 70% and 100% load, zero voltage switching is possible. Input voltage for the aforementioned circuit is 36 v and output power is on the order of 10 w.
It would be advantageous to provide a low cost, full bridge power switching circuit.
It would also be advantageous to provide such a circuit which would require a minimum amount of circuit board space.
Further, it would be advantageous to provide such a circuit capable of zero voltage switching for a range substantially between 0% and 100% load.
It would further be advantageous to provide such a circuit in an off-line converter having a topology that would be effective at high switching frequencies for a range of power of between 50 w to 2 Kw or higher.
It would also be advantageous to provide such a circuit for generating phase modulated square waves.
It would also be advantageous to provide a non-resonating circuit that would perform in accordance with well-known stabilizing techniques.
It would further be advantageous to provide such a circuit having only one phase modulating device and only one one-gate drive transformer.
It would also be advantageous to operate such a circuit to minimize electromagnetic interference and power losses.
It would also be advantageous to provide a switching circuit the components of which need not be rated higher than they would have been if zero voltage switching were not contemplated.
Moreover, it would be advantageous to provide a full bridge circuit having a low failure rate due to few components being subjected to line voltage.